Magnetic media are widely used in various applications, particularly in the computer industry for data/information storage and retrieval applications and in consumer electronics, typically in disk form, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular”, depending upon the orientation of the direction of the magnetic anisotropy of the grains of magnetic material.
Perpendicular recording media have been found to be superior to longitudinal media in achieving very high bit densities without experiencing the thermal stability limit associated with the latter. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high to ultra-high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
Efficient, high bit density recording utilizing a perpendicular magnetic medium requires interposition of a relatively thick (as compared with the magnetic recording layer), magnetically “soft” underlayer (“SUL”), i.e., a magnetic layer having a relatively lower anisotropy of about 1-1,000 Oe, such as of a NiFe alloy (Permalloy), between a non-magnetic substrate, e.g., of glass, aluminum (Al) or an Al-based alloy, and a magnetically “hard” recording layer having relatively high anisotropy, typically about 3-50 kOe, e.g., of a cobalt-based alloy (e.g., a Co—Cr alloy such as CoCrPtB, CoCrPtTaB, etc.) having perpendicular anisotropy. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the magnetically hard perpendicular recording layer.
A typical conventional perpendicular recording system 20 utilizing a vertically oriented magnetic medium 21 with a relatively thick soft magnetic underlayer, a relatively thin hard magnetic recording layer, and a magnetic transducer head 16, is illustrated in FIG. 1, wherein reference numerals 10, 11, 4, 5, and 6, respectively, indicate a non-magnetic substrate, an optional adhesion layer, a soft magnetic underlayer, at least one non-magnetic seed layer (sometimes referred to as an “intermediate” layer or as an “interlayer”), and at least one magnetically hard perpendicular recording layer with its magnetic easy axis substantially perpendicular to the film plane.
Still referring to FIG. 1, reference numerals 7 and 8, respectively, indicate the main (writing) and auxiliary poles of the magnetic transducer head 16. The relatively thin interlayer 5, comprised of one or more layers of nonmagnetic materials, serves to (1) prevent magnetic interaction between the soft underlayer 4 and the at least one hard recording layer 6; and (2) promote desired microstructural and magnetic properties of the at least one magnetically hard recording layer.
As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φ is seen as emanating from the main writing pole 7 of magnetic transducer head 16, entering and passing through the at least one vertically oriented, magnetically hard recording layer 5 in the region below main pole 7, entering and traveling within soft magnetic underlayer (SUL) 3 for a distance, and then exiting therefrom and passing through the at least one perpendicular hard magnetic recording layer 6 in the region below auxiliary pole 8 of transducer head 16. The direction of movement of perpendicular magnetic medium 21 past transducer head 16 is indicated in the figure by the arrow above medium 21.
With continued reference to FIG. 1, vertical lines 9 indicate grain boundaries of polycrystalline layers 5 and 6 of the layer stack constituting medium 21. Magnetically hard main recording layer 6 is formed on interlayer 5, and while the grains of each polycrystalline layer may be of differing widths (as measured in a horizontal direction) represented by a grain size distribution, they are generally in vertical registry (i.e., vertically “correlated” or aligned).
Completing the layer stack is a protective overcoat layer 14, such as of a diamond-like carbon (DLC), formed over hard magnetic layer 6, and a lubricant topcoat layer 15, such as of a perfluoropolyether (PFPE) material, formed over the protective overcoat layer.
Substrate 10 is typically disk-shaped and comprised of a nonmagnetic metal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having a Ni—P plating layer on the deposition surface thereof, or alternatively substrate 10 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials. Optional adhesion layer 11, if present, may comprise an up to about 200 Å thick layer of a material such as Ti, a Ti-based alloy, Cr, or a Cr-based alloy. Soft magnetic underlayer 4 is typically comprised of an about 100 to about 4,000 Å thick layer of a soft magnetic material, which, for example, may be selected from the group consisting of Ni, NiFe (permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFeZrNb, CoFe, Fe, FePt, FeBNi, FeN, FeSiAl, FeSiA1N, FeCoB, FeCoC, etc. Interlayer 5 typically comprises an up to about 300 Å thick layer or layers of non-magnetic material(s), such as Ru, TiCr, RU/CoCr37PT6, RuCR/CoCrPt, etc.; and the at least one magnetically hard perpendicular recording layer 6 is typically comprised of about 50 to about 250 Å thick layer(s) of, for example, Co-based alloy(s) or FePt intermetallic compounds with L10 structure and including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, B, N, C, and Pd.
A currently employed way of classifying magnetic recording media is on the basis by which the magnetic grains of the recording layer are mutually separated, i.e., segregated, in order to physically and magnetically decouple the grains and provide improved media performance characteristics. According to this classification scheme, magnetic media with Co-based alloy magnetic recording layers (e.g., CoCr alloys) are classified into two distinct types: (1) a first type, wherein segregation of the grains occurs by diffusion of Cr atoms of the magnetic layer to the grain boundaries of the layer to form Cr-rich grain boundaries, which diffusion process requires heating of the media substrate during formation (deposition) of the magnetic layer; and (2) a second type, wherein segregation of the grains occurs by formation of oxides, nitrides, and/or carbides at the boundaries between adjacent magnetic grains to form so-called “granular” media, which oxides, nitrides, and/or carbides may be formed by introducing a minor amount of at least one reactive gas containing oxygen, nitrogen, and/or carbon atoms (e.g. O2, N2, C02, etc.) to the inert gas (e.g., Ar) atmosphere during sputter deposition of the Co alloy-based magnetic layer. The latter process does not require heating of the substrate to an elevated temperature.
Magnetic recording media with granular magnetic recording layers possess great potential for achieving very high and ultra-high areal recording densities. An advantage afforded by granular recording layers is significant suppression of media noise due to great reduction in the exchange coupling between adjacent magnetic grains, resulting from the presence of non-magnetic material, typically an oxide material, at the grain boundaries. As indicated above, current methodology for manufacturing granular-type magnetic recording media involves reactive sputtering of a target comprised of the ferromagnetic material for the magnetic recording layer (typically a Co-based alloy) in a reactive gas-containing atmosphere, e.g., an atmosphere comprising oxygen or a compound of oxygen, in order to incorporate oxides in the deposited film or layer and achieve smaller and more isolated magnetic grains. Granular magnetic layers formed in this manner have a reduced saturation magnetization (Ms) due to the oxide formation and consumption of a certain amount of the Co component of the ferromagnetic alloy. Alternatively, a target comprised of the ferromagnetic material (typically a Co-based alloy) and the oxide material may be directly sputtered in an inert atmosphere or an atmosphere comprising oxygen or a compound of oxygen. However, the oxide material sputtered from the target is subject to decomposition in the environment of the sputtering gas plasma, and, as a consequence, a certain amount of the Co component of the ferromagnetic alloy is again consumed.
In order to continually increase the bit density of recording over the next decade, it will be necessary to achieve a corresponding continual decrease of the dimensions of the magnetic grains in order to maintain a good signal-to-noise ration (SNR) of the magnetic media. Therefore, in practice, it will be necessary to decrease the grain volume as the desired linear recording density increases in order to maintain a usable SNR. Such reduction in magnetic grain size, however, will result in grain sizes which approach the so-called superparamagnetic limit of magnetic particles and thereby limit the ability of the media to retain stored information without significant thermal decay. A significant factor with thermal decay associated with grain sizes approaching the superparamagnetic limit is the steepness of onset of the thermal decay. In this regard, it has been estimated that at a certain point a 15% decrease of grain diameter can result in a reduction of storage lifetime of the media from about 20 years to as little as 1 day.
One proposal for overcoming the superparamagnetic limit is to raise the energy barrier to thermal reversal of grain magnetization by development of media with higher anisotropy. However, such approach is problematic in designing high data recording rate media because media with very high coercivities greater than about 10,000 Oe cannot be accurately written to by means of the head fields provided by currently available read/write transducers. This is especially true in high frequency recording applications because of a drastic increase in dynamic anisotropy, resulting in inability of the write field to function at high frequency, leading to incomplete magnetization reversal and causing significant increases in noise level and error rate.
Since the early 1990's, advanced magnetic media have been designed and fabricated for achieving better SNR's. For example, dual layer longitudinal CoNiCr/CoCrTa and dual layer CoCrPt/CoCrPtSi gradient media were fabricated in order to enhance the SNR. Such dual layer media actually are gradient systems wherein the top (upper) layer provides the media with high anisotropy and the lower layer is optimized for reducing media noise.
The ever-increasing need for disk drive media and systems with higher storage capacities, faster data read/write rates, and lower costs form a triad of conflicting and competing requirements for designing, developing, and fabricating the next generation of disk drives. As a consequence, the magnetic recording media design practice faces a number of challenges extending magnetic recording technology to its limits.
Inasmuch as perpendicular magnetic recording media are expected to remain the predominant type of magnetic media for use in disk drives for at least the foreseeable future (e.g., 5-6 years), unique design and engineering schemes are considered necessary for fabrication of improved perpendicular media capable of meeting future challenges and requirements for high recording density, high data recording rate disk drive applications.
In view of the foregoing, there exists a clear need for a new avenue or approach for the engineering and development of advanced perpendicular magnetic recording media which achieves the goals of high linear recording density and high data recording rate without significant loss of thermal stability.
The present invention addresses and solves the need for engineering and development of improved, high performance advanced perpendicular magnetic recording media suitable for use in disk drives, comprising a novel combination of gradient magnetic properties and local vertical exchange coupling, while maintaining full compatibility with all requirements of cost-effective automated fabrication processing.